1. Statement of the Technical Field
The inventive arrangements relate to cellular telephone systems and more particularly to handover of mobile stations from one base station to another.
2. Description of the Related Art
Cellular communication systems generally consist of base stations that transmit and receive radio signals with numerous mobile stations. These communications occur simultaneously on different radio channels of each base station. Conventional base stations commonly employ broad beam antennas to support radio signal coverage over large geographic areas, with each base station intended to cover a different area. The coverage area of a base station will overlap to some extent with the coverage areas of adjacent base stations. These areas of overlap are generally at the outer regions of each base station's coverage areas.
The basic architecture of cellular communication systems and the mobile nature of mobile stations are such that the system does not rely on a fixed set of radio links. Consequently, a call in a cellular communication system is often switched among a plurality of different channels or cells. This process is called handover or handoff.
Due to limited radio spectrum, cellular systems typically employ frequency reuse. This is the repeated use of the same radio frequency channels by multiple base stations throughout a cellular network. The use of the same frequency channels by different mobiles and base stations can cause interference between users of the same frequency channel, known as co-channel interference. To limit co-channel interference, base stations using the same radio channels must be geographically separated by a sufficient distance. A cellular system may employ a frequency reuse of 7, or N=7. This indicates that the same frequency channel may be used in 1 out of 7 base stations in a pattern that attempts to minimize co-channel interference. Frequency reuse limits the overall capacity in a cellular network. The lower the reuse number, the higher the frequency reuse and the greater overall network capacity. For example, a network that has a reuse of N=1 has 7 times the capacity of that using a reuse of N=7. However, in a typical cellular network a frequency reuse of N=1 (or even reuse levels near this level) is not achievable due to co-channel interference.
There are numerous methods that are used to reduce or distribute co-channel interference to allow greater frequency reuse. For example, adaptive antenna arrays can be used to reduce interference by focusing RF energy towards an intended recipient while reducing RF energy directed towards un-intended co-channel users of the same frequency channel. This is accomplished using an array of antenna elements whereby RF energy is electronically steered by adapting the phase and amplitude of radio signals transmitted and received through the antenna array. Adaptive arrays provide significant benefits with regard to addressing the problem of co-channel interference. These systems have the ability to control where the radio signal is received or not received based on spatial properties of the signal.
Adaptive focus and nulling is the process whereby the spatial properties of transmitters received in an uplink signal are analyzed and a solution for the downlink transmission is determined. The desired downlink signal results in a spatial solution that will “focus” RF energy at the geolocation of the intended receiver while reducing or “nulling” RF energy at the geolocations of co-channel users on the same radio channel, thus reducing co-channel interference.
When transmitting an RF signal, a base station utilizing an adaptive array will focus RF energy in a desired direction. The direction is determined by analyzing spatial properties of a received signal coming from the mobile station at the location toward which the base station is to transmit. However, during a handover from one cell to another, the base station targeted for the handover has not received a signal such that it knows where to focus its transmitted energy. If the targeted base station does not know where to transmit its energy, handover failures will occur. Such handover failures will result in dropped calls.